A digital-to-analog converter (DAC) is employed to output an analog signal which is directly applicable to analog equipment such as speakers. However to provide an output signal of the DAC to analog equipment, a buffer circuit is required to supply enough electric current drive the equipment.
FIG. 1 shows conventional buffer circuits.
Both of the bases of a first npn-bipolar transistor Q1 and a second pnp-bipolar transistor Q2 are connected to an input node 11. A collector of the first transistor Q1 is connected to a first power supply node 12, and an emitter of the first transistor Q1 is connected to a second power supply node 13 via a first constant current source I.sub.1. An emitter of the second transistor Q2 is connected to the first power supply node 12 via a second current source I.sub.2, and a collector of the second transistor Q2 is connected to the second power supply node 13.
A base, collector, and emitter of a third npn-bipolar transistor Q3 are connected to the emitter of the second transistor Q2, the node I.sub.2, and an output node 14, respectively. A base, collector, and emitter of a fourth pnp-bipolar transistor Q4 are connected to the emitter of the first transistor Q1, the second power supply node I.sub.3, and the output node 14, respectively.
FIG. 2 shows a relationship between an input signal V.sub.in and output signal V.sub.out of the buffer circuit shown in FIG. 1.
First, when a level of the input signal V.sub.in is between V.sub.BE and V.sub.CC -V.sub.BE with reference to a ground level (0V), all of the transistors Q1, Q2, Q3, and Q4 are in a conductive state. Therefore, the output signal V.sub.out varies linearly in accordance with the input signal V.sub.in. V.sub.CC represents a power supply voltage, and V.sub.BE represents a threshold voltage of bipolar transistors.
Second, when the level of the input signal V.sub.in is higher than V.sub.CC -V.sub.BE with reference to the ground level (0V), a voltage across the base and emitter of the second transistor Q2 is lower than the threshold voltage V.sub.BE, and the second transistor Q2 is in a non-conductive state. Therefore, current of the second constant current source I.sub.2 flows into the base of the third transistor Q3, and the third transistor Q3 remains in the conductive state. A voltage across the base and emitter of the fourth transistor Q4 is lower than the threshold voltage V.sub.BE, and the fourth transistor Q4 is in a non-conductive state. Therefore, the output signal V.sub.out is clipped at a level of V.sub.CC -V.sub.BE.
Finally, when the level of the input signal V.sub.in is lower than V.sub.BE with reference to a ground level (0V), a voltage across the base and emitter of the first transistor Q1 is lower than the threshold voltage V.sub.BE, and the first transistor Q1 is in a non-conductive state. Therefore, all the current of the first constant current source I.sub.1 flows into the base of the fourth transistor Q4, and the fourth transistor Q4 remains in the conductive state. A voltage across the base and emitter of the third transistor Q3 is lower than the threshold voltage V.sub.BE, and the third transistor Q3 is in the non-conductive state. Therefore, the output signal V.sub.out is clipped at a level of V.sub.BE.
In the conventional buffer circuit, as described above, when the level of the input signal V.sub.in is lower than the threshold voltage V.sub.BE, the output signal V.sub.out is clipped at the level of V.sub.BE, and when the level of the input signal V.sub.in is higher than V.sub.CC -V.sub.BE, the output signal V.sub.out is clipped at the level of V.sub.CC -V.sub.BE. Namely, a dynamic range of the output signal is .vertline.V.sub.CC -2.times.V.sub.BE .vertline..
It is well known that the typical value for V.sub.BE of a bipolar transistor is 0.7V. Therefore, if the power supply voltage V.sub.CC is 3V, the dynamic range of the output signal is 1.6V (=3V-2.times.0.7V).
The narrow dynamic range leads to a distortion in the output signal. The distortion may be eliminated by attenuating the input signal. However, attenuating the input signal leads to deterioration in signal-to-noise (S/N) ratio. These problems are serious in low-supply-voltage circuits.